Siem Stefan de Jong (Dutch pronunciation: ; born 28 January 1989) is a Dutch professional footballer who plays as an attacking midfielder or striker for Sydney FC on loan from Ajax.

Playing in the youth academies of De Graafschap and Ajax, he made his professional debut for the Amsterdam-based side and would go on to become the club's captain as they won a club-record four consecutive Eredivisie titles from 2011 to 2014.

Following a three-year spell with Premier League side Newcastle United and a loan spell with PSV Eindhoven, De Jong returned to Ajax in 2017.